Simulation Study Of Lean Burn Gasoline Engine Nox Pollutants Reduction Using Lean Nox Trap

The major concern for the development of advanced internal combustion engine technology involves better fuel economy and lower emission reduction. Lean burn technology for gasoline engines, including both direct injection stratified charge (DISC) and port fuel injection (PFI) engines, has drawn great attention during the past two decades, largely due to its potential for improving fuel economy and reducing harmful pollutants and greenhouse gas emissions.However, a challenge for lean burn technologies is that, under lean operating conditions, the conventional three-way-catalyst (TWC) system is no longer effective in reducing NOx pollutants. Currently, Lean NOx Trap (LNT) is more efficient mothed to reduce lean burn gasoline engine NOx pollutants. When its stored NOx reaches a certain threshold, the LNT must be purged by switching to rich operation for a short period of time to regenerate the storage sites and recover the efficiency. When exposed to the rich exhaust environment, the nitrate becomes thermodynamically unstable and the stored NOx is released from the NOx storage sites. The released NOx is then catalytically reduced by reductant such as CO, HC and H2 in the feedgas. To achieve the best tradeoff among competing requirements such as fuel economy, emissions and driveability, the LNT control strategy must manage the purge starting time and duration, and at the same time provide a bumpless transition between the lean and purge modes.In this paper, the author set up control-oriented engine model as well as LNT model and successfully calculates the LNT NOx volume storage percentage. This storage percentage value was transmitted into engine model as a parameter for engine lean and rich combustion mode automatic switch. The entire engine model consist of ETC Model, Engine Gas Dynamics Subsystem, EGR Model, Fuel Rate Calculation and Control Logic Subsystem, Torque Generation and Control Subsystem,LNT Subsystem and data display block. The LNT control strategy simulation as well as engine speed and torque control during transition period has realized by combination operation of control-oriented engine model and LNT model. In addition, through input NOx emission experiment data to LNT model, the simulation display the relationship between NOx maximum storage threshold of LNT and NOx flow out of LNT as well as NOx stored in LNT during transition period. The result shows that on the condition of the same LNT volume, the lower NOx maximum storage threshold setting will bring more frequently transition between lean and rich modes, in the same time the lower NOx flow out of the LNT during transition period and the better time interval between lean and rich operation condition.